In the art of petroleum production, a borehole is drilled into the earth through the oil or gas producing subsurface formation or, for some purposes, through a water bearing formation or a formation into which water or gas is to be injected. Completion of a well may be carried out in a number of ways dependent upon the nature of the formation of interest. For example, where a relatively stable, consolidated earth formation is penetrated, oil may be produced or fluids injected through an open hole. On the other hand, where the formation itself or formations above the formation of interest have a tendency to disintegrate and/or cave into the hole, a casing is normally set in the well through the formation of interest and the casing is then perforated adjacent the formation of interest. Finally, where the formation of interest is a substantially unconsolidated sand, which has a tendency to flow into the well along with the produced fluids, a slotted liner or screen is hung inside a casing or oil string opposite the producing zone. The oil string may have been either cemented through the section and perforated or set on top of the section with the liner hanging below the casing and through the formation. In some instances, the well bore may be slightly enlarged in the formation of interest, packed with gravel and then have a slotted liner or screen set in the gravel pack. In any event, after a period of production of fluids or the injection of fluids, there is a tendency for the openings in the casing or liner and/or the pores of the formation itself to become plugged with various types of residues. For example, paraffin, asphalts and other gummy residues of petroleum origin often cause plugging problems. While such deposits are not in and of themselves a serious problem, because of their relatively soft nature and the fact that they can be dissolved rather readily with certain chemicals, these materials of petroleum origin either together with chemicals from water, normally produced with the oil, such as, calcium sulfate and the like, or such chemicals themselves have a tendency to form extremely hard deposits, particularly, in the slots of a liner, the openings of a screen or the perforations of a well casing. These hard materials adhere to the casing, liner or screen, restricting the openings and, thus, seriously reducing or completely preventing the flow of fluids through the openings. These hard encrustations are extremely difficult to dissolve by known chemical means or to dislodge by known mechanical means. Consequently, it is often necessary to rework the well and replace the liner or reperforate the casing. Such tactics are, of course, both time-consuming and expensive.
Chemical treatments, such as, treatments with acids, surface active agents and the like have been utilized in order to clean out plugged openings in the casing, liner or screen. However, such techniques, while less expensive than a complete workover, are substantially less effective, since they are incapable, in most cases, of dissolving significant amounts of the plugging materials.
Numerous techniques, which can be classified as mechanical techniques, have also been suggested for the purpose of cleaning openings in the casing, liner or screen. These include applying heat by an electrical heater, the ignition of gas in an electric arc, etc. However, the heat generated in these instances is generally insufficient to have any real effect even on materials, such as, paraffin, asphalt, etc. which can be liquified by heat. The primary difficulty, however, is that the well is normally full of liquids, under a high pressure and the heat of such heating means is rapidly dissipated in the well fluid before it has an opportunity to have any real effect on deposits in the casing or liner.
It has also been proposed to create shock waves adjacent the formation to thus dislodge such deposits. One means which has been proposed is the ignition of a combustible gas or gasses by means of an electric arc with the thought that sufficient heat and/or a mechanical shock wave will be created. However, such a source of heat and/or shock waves is generally of too low an intensity to have any appreciable effect in cleaning the openings in the casing, liner or screen. To date, no practical means for overcoming these problems has been suggested.
Another means of mechanically cleaning openings in a casing, liner or screen which has received some attention in recent years is the creation of ultrasonic vibrations in the well bore. The interest in this technique stems from the fact that ultrasonic irradiation can create cavitation in a liquid medium. Specifically, the ultrasonic irradiation causes the formation of local cavities in a liquid, as a result of the reduction of total pressure. When these cavities collapse, they produce very large impulse pressures. However, in order to accomplish these results, a very high source of power must be utilized. While numerous efforts have been made to develop high power, ultrasonic generators, only limited success has been achieved and this success has not been sufficient to adequately clean well screens or liners, even under ideal laboratory conditions. Moreover, under normal hydrostatic pressures encountered in a well fluid, i.e. 200 psig, it is difficult, if not impossible, to achieve cavitation with presently available ultrasonic transducers. Here again, no practical solutions have been found.
It is also desirable, either on newly drilled wells or on wells that have been utilized for some time for production or for water or gas injection, to resort to various well stimulation techniques to remove plugging deposits from the pores of the formation surrounding the well bore or to actually enlarge the flow channels through the formation of interest. It is highly desirable to maintain the formation immediately surrounding the well in a highly permeable condition since the formation immediately adjacent the well bore has a profound effect on the ability to produce fluids from the formation or inject fluids into the formation. This follows from the obvious fact that fluids are being produced from an extremely large volume of the formation at some distance from the well bore and these fluids then must be funnelled through or produced through the very limited volume of formation surrounding the well bore. Thus, plugging of the area immediately surrounding the well bore has a drastic effect upon the resistance to flow of the fluids. All of the above-mentioned techniques for cleaning casing perforations, liners, screens and the like have also been utilized with some success for removing plugging deposits from earth formations and/or increasing the permeability of the formation. For example, conventional acidizing, mud acid treatments, jet acidization and the use of surface active agents have been utilized. However, in this instance, the treatment is generally no more successful than the treatment to clean out the openings in the casing or liner. Acidization has also been utilized to actually increase the permeability of a formation of interest. However, a more effective technique for increasing permeability has been that of hydraulic fracturing. In this instance, a relatively viscous fluid is pumped into the formation of interest at a pressure and for a time sufficient to actually fracture the formation. Thereafter, sand is carried into the fractures by means of an appropriate carrier fluid and the pressure is then released to permit the fracture to partially close and hold the sand in place. While this technique has proven to be highly successful in improving formation permeability, it is a relatively costly operation and it is generally not utilized where only plugging is the cause of restricted flow or where permeability improvement is necessary only in a very limited section of the formation surrounding the well bore.
It is therefore an object of the present invention to provide an improved method and apparatus for treating boreholes extending into the earth. Another and further object of the present invention is to provide an improved method and apparatus for cleaning openings in casings, liners, screens, etc. positioned adjacent a subsurface borehole. Yet another object of the present invention is to provide an improved method and apparatus for increasing the production of fluids from a subsurface earth formation or increasing the injectivity of fluids into such formations. A still further object of the present invention is to provide an improved method and apparatus for increasing the effectiveness of chemical treating agents utilized in cleaning openings in casings, liners, screens, etc. positioned adjacent a subsurface earth formation. A further object of the present invention is to provide an improved method and apparatus for increasing the effectiveness of chemicals utilized in the treatment of subsurface earth formations to increase the permeability thereof.